Michel,
at least one use-case was forgotten when we designed bibbackend interfaces. Here it is:
1. Configure sources for using Biblatex/Biber
2. Compile (Kile stores autodetected backend)
3. Change sources to use BibTeX
Now Kile uses autodetected Biber and compilation fails.
Proposed patch adds an menu item to reset stored autodetection tool. This returns state of tools to its origin. Thus it might cover another forgotten use-cases.
P.S. Perhaps action title can be shorter. Can not find such a formulation.

We can have an explicit reset action but I think it would be better if the auto-detected tool is cleared as soon as a bibtool is selected manually.
Also, could you add the reset action to the "Bibliography Back End" menu itself (at the end, or so)?